The efficiency of Cathode Ray Tube (CRT) phosphors normally decreases with use. If the scan is moved or changed in any way during use, the brightness of the phosphor will vary over the used scan area in a random way giving rise to what is called a burn pattern.
It is very difficult, if not impossible, to manufacture a CRT phosphor screen with no blemishes and an insignificant grain structure. Even after careful processing and selection, there will always remain a few blemishes of small size and amplitude, and a detectable grain structure. For the purposes of this specification, burn correction should be regarded as correcting for errors originating from the phosphor screen such as phosphor burn, grain pattern and blemishes.
The above errors can be removed by a burn corrector, and one well known method uses a photomultiplier to measure or monitor the CRT brightness and uses the signal so generated to modulate the video signals and correct the error.
A further problem affecting signal integrity is dirt on the CRT faceplate. Even though many precautions are taken to eliminate dirt, one speck of dirt can cause annoyance.
The dirt on the faceplate is displaced from the phosphor by the thickness of the faceplate. It is therefore necessary that the burn and dirt detector are on the same optical axis as the film scanning axis to ensure correct spatial registration of the dirt and burn correcting signal with the main signal it is going to modulate.